Various approaches have been developed to allow multiple users to reuse a single timeslot in time slotted wireless systems, referred to as Multiple Users Reusing One Slot (MUROS) technologies or Voice Services Over Adaptive Multiuser Channels On One Slot (VAMOS). One such approach involves the use of orthogonal sub-channels (OSC). The OSC concept allows a wireless network to multiplex two wireless transmit/receive units (WTRUs) that are allocated the same radio resource (that is, time slot) and Global System for Mobile communication (GSM) channel, thus the capacity may be significantly improved for a number of available transceiver (TRX) hardware and possibly for the spectrum resource. Furthermore, such a feature is expected to provide voice capacity improvement for both full rate and half rate channels.
In the uplink (UL) direction, the sub-channels are separated using non-correlated training sequences. The first sub-channel uses existing training sequences, and the second sub-channel uses new training sequences. Alternatively, only new training sequences may be used on both of the sub-channels. Using OSC enhances voice capacity with negligible impact to WTRUs and networks. OSC may be transparently applied for all Gaussian minimum shift keying (GMSK) modulated traffic channels (for example, for full rate traffic channels (TCH/F), half rate traffic channels (TCH/H), a related slow associated control channel (SACCH), and a fast associated control channel (FACCH)).
OSC increases voice capacity by allocating two circuit switched voice channels (that is, two separate calls) to the same radio resource. By changing the modulation of the signal from GMSK to quadrature phase shift keying (QPSK) (where one modulated symbol represents two bits), it is relatively easy to separate two users—one user on the X axis of the QPSK constellation and a second user on the Y axis of the QPSK constellation. A single signal contains information for two different users, each user allocated their own sub-channel.
In the downlink (DL), OSC is realized in a base station (BS) using a QPSK constellation that may be, for example, a subset of an 8-PSK constellation used for enhanced general packet radio service (EGPRS). Modulated bits are mapped to QPSK symbols (“dibits”) so that the first sub-channel (OSC-0) is mapped to the most significant bit (MSB) and the second sub-channel (OSC-1) is mapped to the least significant bit (LSB). Both sub-channels may use individual ciphering algorithms, such as A5/1, A5/2 or A5/3. Several options for symbol rotation may be considered and optimized by different criteria. For instance, a symbol rotation of 3π/8 would correspond to EGPRS, a symbol rotation of π/4 would correspond to π/4-QPSK, and a symbol rotation of π/2 may provide sub-channels to imitate GMSK. Alternatively, the QPSK signal constellation may be designed such that it resembles a legacy GMSK modulated symbol sequence on at least one sub-channel.
Several reasons favor QPSK as a choice for the MUROS/VAMOS modulation format. First, QPSK offers robust signal-to-noise ratio (SNR) vs. bit error rate (BER) performance. Second, QPSK may be realized through existing 8-PSK-capable RF hardware. Third, QPSK burst formats have been introduced for Release 7 EGPRS-2 for Packet-Switched Services.
An alternate approach of implementing MUROS/VAMOS in the downlink involves multiplexing two WTRUs by transmitting two individual GMSK-modulated bursts per timeslot. As this approach causes increased levels of inter-symbol interference (ISI), an interference-cancelling technology such as Downlink Advanced Receiver Performance (DARP) Phase I or Phase II is required in the receivers. Typically, during the OSC mode of operation, a base station (BS) applies DL and UL power control with a dynamic channel allocation (DCA) scheme to keep the difference of received downlink and/or uplink signal levels of co-assigned sub-channels within, for example, a ±10 dB window. The targeted value may depend on the type of receivers multiplexed and other criteria. In the uplink, each WTRU may use a normal GMSK transmitter with an appropriate training sequence. The BS may employ interference cancellation or joint detection type of receivers, such as a space time interference rejection combining (STIRC) receiver or a successive interference cancellation (SIC) receiver, to receive the orthogonal sub-channels used by different WTRUs.
OSC may be used in conjunction with frequency-hopping or user diversity schemes, either in the DL, in the UL, or both. For example, on a per-frame basis, the sub-channels may be allocated to different pairings of users, and pairings on a per-timeslot basis may recur in patterns over prolonged period of times, such as several frame periods or block periods.
Statistical multiplexing may be used to allow more than two WTRUs to transmit using two available sub-channels. For example, four WTRUs may transmit and receive speech signals over a 6-frame period by using one of two sub-channels in assigned frames.
An extension of the baseline concept called the a-QPSK modulation scheme has been introduced. The a-QPSK modulation scheme suggests a simple means of power control for the in-band and quadrature components of the QPSK symbol constellation. By using an a parameter, the relative power on the MUROS/VAMOS timeslot allocated to the first vs. the second sub-channel on the timeslot may be adjusted in a range of ±10-15 dB relative to each other. Using this approach, the absolute power allocated by the transmitter to the composite MUROS/VAMOS transmission no longer needs to be precisely ½ power for each user (equivalent to relative power of sub-channel 1/power sub-channel 2 at 0 dB). Other more desirable power ratios may be achieved, such as when one of the MUROS/VAMOS sub-channel (user) is in better signal conditions than the other user, and a power ratio of −3 dB (or higher) would result in better performance for the weaker MUROS/VAMOS user. Together with the absolute Tx power setting of the MUROS/VAMOS composite signal on the timeslot, the α-QPSK concept would result in a relative power control component for MUROS/VAMOS users.
Another possible extension of this baseline OSC proposal suggests multiplexing of more than just a simple fixed pair of users into the very same allocated burst in all frames by extending the concept to statistical multiplexing of more than just 2 users over a period of at least several frames in a GSM multi-frame structure. At any given point in time (that is, any “burst”), not more than 2 users will transmit using the 2 available sub-channels of the OSC burst. However, when using Half Rate (HR) codecs (any WTRU required to Tx/Rx 1 out of 2 frames), statistical multiplexing of more than just 2 users can be achieved. For example, four users can Tx/Rx their HR speech signals over any given 6 frame period using one of the two available OSCs per burst, and by transmitting only in their assigned frames.
An even further possible modification to the baseline OSC proposal suggests that re-use of GSM Frequency-Hopping (FH) techniques would result in both interference averaging and the discontinuous transmission (DTX) gains for OSC and non-OSC users, with gains spread relatively equally amongst the WTRUs in the cell. Similar to the first possible modification, in any given burst (i.e., timeslot) not more than 2 users will transmit using the 2 available sub-channels of the OSC burst. However, by assigning different frequency-hopping sequences/Mobile-Allocation-Index-Offset's (MAIO's) to the different WTRUs in the cell, any WTRU may be paired with another WTRU on the next occurrence of a burst. The pattern would repeat after a certain number of frames, as a function of the FH-list. Note that this is applicable to both DL and UL directions.
With regard to the UL direction, the MUROS/VAMOS proposals and/or extensions including the Frequency-Hopping concept for statistical multiplexing handsets suggest using normal GMSK transmission with different training sequences on the same time slot to allow the BS to distinguish between the two transmissions. Each of the two handsets would transmit a legacy GMSK modulated burst, unlike the OSC DL which may use QPSK. It is assumed that the BS uses either STIRC or SIC receiver to receive orthogonal sub channels used by different WTRUs.
The above mentioned proposals are not mutually exclusive. These proposals only differ in how to achieve the goals for MUROS/VAMOS using either existing functionality, or through introducing new capability into the WTRU design.
With respect to the aforementioned second technical proposal referring to Release 6 DARP-type I receiver implementations in handsets, MUROS/VAMOS suggests that speech services may be provided to two users simultaneously over the same physical channel, or timeslot. One of these multiplexed users can be a legacy user. The legacy WTRU could be either with or without single antenna interference cancelation (SAIC) or DARP support implemented. Similarly, a new type of MUROS/VAMOS equipment will rely on DARP-like interference-type cancelation receivers. In addition, new MUROS/VAMOS equipment may be expected to support features like extended training sequences.
According to existing GSM specifications, once a Traffic Channel Full Rate (TCH/F) is assigned to a WTRU, the BS and the WTRU will start communicating with each other, at the physical layer, according to a 26-Frame Multiframe protocol. In order to convey radio related parameters and signaling, a TCH is always associated with a Slow Associated Control Channel (SACCH). Moreover, there also exists a Fast Associated Control Channel (FACCH) to convey service related signaling between the WTRU and the network. Typical messages on SACCH are System Information in DL and Measurement Report in UL. The FACCH is normally used for Handover as well as Assignment messages when the WTRU is operating on a TCH. The WTRU may also operate in a Stealing Mode such that, when needed, it may steal from traffic resources and using them for signaling purposes.
FIG. 1 illustrates the mapping of TCH and SACCH on a 26-Frame multiframe according to the existing GSM standards. It should be noted also that due to the nature of the half rate configuration, the same observations as for TCH/F also apply to the half rate configurations. There is one occurrence of SACCH and one occurrence of an Idle frame per multiframe. In MUROS/VAMOS operation, each of the two (or more) WTRUs multiplexed onto a Time Slot will still need to follow the mandated multiframe configuration.
Due to the robust coding and decoding performance of MUROS/VAMOS receivers on traffic channels, the associated control channels (i.e. SACCH and FACCH) interlace into the speech multi-frame and become un-decodable well before the actual speech bursts. It is of great importance to realize that in legacy GSM networks, the actual link between the WTRU and the BS is supervised by the SACCH according to a well-known radio link failure counter in GSM called Radio Link Timeout (RLT). This means that an entity (a WTRU or BS) shall release an active connection not when the actual speech burst decoding quality degrades below an unacceptable threshold, but rather upon successive failure of decoding a SACCH. Note that the actual RLT value is signaled by the network to the WTRU. Therefore, with the advent of MUROS/VAMOS, the decoding performance of the associated control channels in the speech multi-frames and their intimate linking into the radio link failure criterion constitutes the limiting factor. Accordingly, it is necessary to improve the performance of the SACCH to allow for MUROS/VAMOS operation even in weak signal or strong interference conditions.